To date, there are known various types of internal combustion engines having pre-chamber spark plugs. In one such example, the engine includes: a housing with a cylinder and a pre-chamber spark plug; the pre-chamber spark plug has a body, an external thread at its front end, a passage, an insulator positioned in the passage, a center electrode protruding from the front end of the insulator, and a pre-chamber-forming cap, which is placed at the front end of the body and delimits a pre-chamber; and the cap shields the center electrode from the combustion chamber and has an opening, which enables a gas exchange between the pre-chamber and the combustion chamber.
SUMMARY
An object of the present design is to improve an internal combustion engine and/or a pre-chamber spark plug of the type mentioned above.
This object may be attained by means of an internal combustion engine with different combinations of features, such as those listed in the claims. The internal combustion engine according to the present design has a housing with at least one cylinder and a pre-chamber spark plug. The housing can include an engine block and a cylinder head. The cylinder has a piston that is able to move in the housing and delimits a combustion chamber contained in the housing. The volume of the combustion chamber changes as the piston moves. The volume of the combustion chamber when the piston is at top dead center is referred to as the “compressed volume.” The pre-chamber spark plug has a body with an external thread on its front end. The body has a passage in which an insulator is fastened and a center electrode protrudes from the front end of this insulator. The front end of the insulator is also referred to as the “insulator foot.” The center electrode can form a spark gap with the inner wall of the pre-chamber. Alternatively, at least one ground electrode can be provided, which is connected to the body in an electrically conductive fashion and forms a spark gap with the center electrode. At the front end of the body, a pre-chamber-forming cap is provided, which delimits a pre-chamber. The cap shields the center electrode—and if present, also the ground electrode—from the combustion chamber and has at least one opening, which permits a gas exchange between the pre-chamber and the combustion chamber. It can be a passive pre-chamber spark plug in which the pre-chamber is supplied with fuel exclusively from the combustion chamber of the internal combustion engine, namely via the at least one opening in the cap. In particular, the passive pre-chamber spark plug does not contain any fuel supply conduits that supply additional fuel directly to the pre-chamber. According to one embodiment, the total volume of the pre-chamber makes up at least 0.65% of the compressed volume of the combustion chamber. The total volume of the pre-chamber can lie in the range from 0.65% to 1.9%, in particular 0.7% to 1%, of the compressed volume of the combustion chamber.
The present design may enjoy the following advantages:                In gasoline-powered spark-ignition engines, the present design can improve lean-burn operation and enable a reliable, large-volume ignition in the combustion chamber by means of ignition torches or flame jets that shoot out from the openings in the cap, in particular even when the exhaust recirculation rate is relatively high.        The piston displacement of the cylinder of the internal combustion engine according to the present design can lie in the range from 300 cm3 to 800 cm3, in particular from 300 cm3 to 500 cm3. It has surprisingly turned out that especially with a piston displacement of this kind, a particularly good ignition can be achieved if the total volume of the pre-chamber is chosen to be relatively large and makes up at least 0.65% of the compressed volume of the combustion chamber. With a compressed volume of the combustion chamber in the range from 30 cm3 to 100 cm3, in particular from 50 cm3 to 80 cm3, this can be achieved with a total volume of the pre-chamber in the range from 0.2 cm3 to 1 cm3, in particular from 0.3 cm3 to 0.8 cm3.        
In another embodiment, the internal combustion engine can have a plurality of cylinders. Each cylinder has a combustion chamber, a conduit feeding into the latter, and a pre-chamber spark plug. In all of the pre-chamber spark plugs of the internal combustion engine, the opening in the cap can have the same orientation in relation to the conduit. In all of the cylinders of an internal combustion engine, this can ensure a defined orientation of the flame jet—which shoots out from the opening during the ignition event—in the combustion chamber and can improve the ignition of a lean mixture therein. The opening in the cap can be oriented obliquely to the center line of the external thread. In particular, the center line of the opening, especially the center line of each of the openings in a cap, can intersect the center line of the external thread. This can produce an essentially swirl-free, high-turbulence flow in the pre-chamber, thus making it possible to ensure a good flushing of the pre-chamber and a reliable ignition. The high turbulence or high “turbulent kinetic energy” also ensures an acceleration of the combustion. The angle between the center line of the opening and the center line of the external thread can be from 30° to 80°, in particular from 45° to 60°. The cap can have 3 to 9 openings, in particular 4 to 6 of them. The at least one opening can have a diameter of 0.6 mm to 1.6 mm, in particular 0.8 mm to 1.4 mm.
In the body, an annular seat surface for the insulator can be provided at which the passage narrows, viewed from the back end to the front end. The front end of the insulator, i.e. the insulator foot, protrudes forward beyond the seat surface into the pre-chamber and is spaced apart from the body by a spacing distance of at least 1.2 mm extending in the circumference direction. The spacing distance between the front end of the insulator and the pre-chamber wall can in particular be 1.4 mm or more. When viewed from the back to the front, the passage can narrow at a point situated between the annular seat surface for the insulator and the fastening point of the ground electrode. The constriction situated between the annular seat surface for the insulator and the ground electrode, in particular the fastening point of the ground electrode on the body, can enable an improved fastening of the ground electrode. The ground electrode can contact the body along more of its length. It is thus possible to lengthen the weld between the ground electrode and the body. The constriction can in particular protrude into the pre-chamber “like a balcony” behind the ground electrode. The passage can have its smallest free cross-section at the constriction that supports the ground electrode. The passage of the body can thus narrow at two points; it is also possible for the passage to widen between the two constrictions. The passage—viewed from the back to the front—can widen at a point situated between the annular seat surface for the insulator and the front end of the insulator, i.e. in the vicinity of the insulator foot. In the vicinity of the insulator foot, an open annular space can be produced, which is large enough to ensure a good flushing of the pre-chamber.
In another embodiment, the pre-chamber can be divided by an imaginary dividing plane into a front part and a back part. The dividing plane extends perpendicular to the center line of the external thread and is positioned at an end surface of the center electrode that protrudes from the insulator. The front part of the pre-chamber is situated on the side of the dividing plane oriented toward the front end of the spark plug and the back part of the pre-chamber is situated on the side of the dividing plane oriented toward the back end of the spark plug. The back part of the pre-chamber is situated inside the spark plug, in particular inside the body. Since the dividing plane only conceptually divides the pre-chamber into two parts, they remain connected to each other at the dividing plane. Apart from this connection of the back part of the pre-chamber to the front part of the pre-chamber at the dividing plane, the back part of the pre-chamber is closed in a gastight fashion. “Gastight” means that aside from the gas exchange with the front part of the pre-chamber taking place at the dividing plane, no gases can escape from the back part of the pre-chamber during operation. The volume of the back part of the pre-chamber is greater than the volume of the front part of the pre-chamber. The volume of the back part of the pre-chamber can be greater than the volume of the front part of the pre-chamber by a factor of 1.5 to 2.0, in particular by a factor of 1.6 to 1.7. This can achieve an enlarged space downstream of the spark gap into which residual gases from the previous power stroke of the engine can be displaced during a compression stroke. Thanks to this enlarged storage space for residual gases, a practically undiluted fresh fuel/air mixture can be present at the ignition gap between the center electrode and the ground electrode, thus enabling improved ignition by the sparks.